The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. Replacement of traditional carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also possible because inert anodes produce essentially no CO2 or CF4 emissions. Some examples of inert anode compositions are provided, for example, in U.S. Pat. Nos. 5,794,112 and 5,865,980, assigned to the assignee of the present application. These patents are incorporated herein by reference.
A significant challenge to the commercialization of inert anode technology is the anode material. Researchers have been searching for suitable inert anode materials since the early years of the Hall-Heroult process. The anode material must satisfy a number of very difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte. It must not react with oxygen or corrode in an oxygen-containing atmosphere. It should be thermally stable at temperatures of about 1,000° C. It must be relatively inexpensive and should have good mechanical strength. It must have high electrical conductivity at the smelting cell operating temperatures, of about 900° C. to 1,000° C., so that the voltage drop at the anode is low.
Oxides that are particularly well suited for processing into advanced inert, non-consumable and dimensionally stable anodes, such as, for example, MxFe3−xO4±δ, where x from 0 to 3, M represents one or more elements selected from at least one of the group of Ni, Cu, Co, Zn, Cr, Mn, Al, Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Pd, Ag, Ca, Sr, or Sn, respectively, and δ is a variable dependent on process conditions. The oxides are used alone or with a metal phase including copper and/or at least one noble metal, and are usually made by conventional solid-state reaction techniques, which require repeat ball milling of calcined oxide powders. This conventional method often produces chemically inhomogeneous oxide powders with larger particle sizes and wide size distribution. The chemically inhomogeneous oxides can cause second phases present on fired parts that affect their performance. Large particle size requires time-consuming grinding/milling. For example, U.S. Pat. Nos. 5,794,112 and 5,865,980 (both Ray et al.) taught ball milling a mixture of NiO and Fe2O3 that had already been mixed, ground and calcined for 12 hours at 1250° C. Particles of about 10 micrometers average particle size were the end product after about 40 hr. ball milling. FIG. 1 shows a representation of 40 hr. ball milled particles using conventional methods. Besides composition inhomogenity, the wide distribution of particle size may lead to high porosity and less uniform microstructure due to non-uniform grain growth during sintering. These particles were blended with water and polymeric binder and spray dried, V-blended and pressed into an anode shape for sintering. U.S. Pat. No. 6,217,739 B1 (Ray et al.) taught Fe2O3, NiO and ZnO inert anode starting materials where a metal phase could comprise copper and/or silver and the like. The inert anodes could be formed by standard powder forming, sol-gel processing, slip casting, coating or hot pressing, preferably powder techniques, which were emphasized in the patent.
Djega-Marinadassou et., in U.S. Pat. No. 4,894,185 taught making zinc oxide based powder for varistors by providing a mixture of zinc nitrate or zinc chloride with a minor amount of, for example, cobalt nitrate to provide a mixed solution in water and then providing a bismuth nitrate or lead nitrate solution. Both solutions were added to a given volume of ammonia buffer solution, pre-saturated by cations of the elements to be precipitated—zinc and cobalt, kept at a defined pH, leading to precipitation of the hydroxides to provide a co-precipitate. The co-precipitate is then filtered, dried at room temperature, and finally calcined at 700° C. to provide a homogeneous oxide material. U.S. Pat. No. 5,290,759 (Richardson et al.) taught Y Ba2Cu3O6+x metal oxide superconductors by co-precipitation of respective nitrates using alkali hydroxide as a precipitating agent, followed by filtration, washing in the presence of CO2, drying, firing and cooling.
An earlier process by Goldman et al., in U.S. Pat. No. 4,097,392 taught manufacturing ferrimagnetic material, such as manganese-zinc ferrites, for magnetic ceramics to provide a more homogeneous product. There, pure metals were used as the starting materials in making the aqueous metal ion solution rather than salts of the metals, where other metals such as zinc, manganese, nickel and magnesium can be added to the iron metal ions. The aqueous metal ion solution is then reacted with an ammonium, sodium or potassium carbonate solution to concurrently co-precipitate ferrous hydroxide and one of the other metals without conversion to ferric ions in a manner to provide a selected ratio between carbonate and hydroxide groups. The co-precipitated material is then separated from the liquid phase and dried. U.S. Pat. No. 5,788,950 (Imamura et al.) is another patent in this area, where ceramic oxide provides for filters, capacitors or oxygen sensors are synthesized. There, a liquid absorbent resin is combined with solutions containing organo-metallic compounds and solutions containing metallic salt compounds. The resin then swells and gels. A precursor material is then prepared by changing the pH and/or temperature of the swollen gel. After pyrolyzing and calcining, a mixed metal oxide powder is formed, providing more homogeneous, smaller sized powders than either co-precipitation or repeated ball milling and providing a much less expensive process than sol-gel routes which usually involve use of very expensive precursors.
While all of these approaches have various advantages, in methods by Djega-Mariadassou et al. and Goldman et al., there are three drawbacks: 1) because different metals tend to precipitate out at different pH, it causes non-homogeneous precipitates which will give non-homogeneous final products; 2) the precipitate needs to be separated/filtered from solution. There are always residual metal cations left in the solution that usually lead to final composition shift; 3) if alkali hydroxides/carbonates are used, the residual alkali cation will contaminate the final products. Method by Imamura et al. used resin to form a gel then pyrolyze and calcine the gel. The large amount of resin not only increases the cost but also causes CO/CO2 during pyrolyzing and calcining. What is needed is a simpler approach than that of Inamura et al. while still solving the problems of co-precipitation and sol-gel processing cited by Imamura et al.
The present invention has been developed in view of the foregoing and to address other deficiencies of the prior art.